1. Field of the Invention
The present invention relates to a multiplex communication system, and more particularly, to an automotive multiplex communication system having a sleep control function.
2. Description of the Related Art
An automobile mounted with electrical equipment units such as sensors, electronic control units (ECUs), actuators is generally provided with a multiplex communication system for data transfer between the electrical equipment units. The multiplex communication system is comprised of a multiplex transmission path and communication nodes connected thereto, and one or more electrical equipment units are connected to each communication node. In the multiplex communication system, communication nodes are rendered operative when supplied with electric power, making it possible to carry out data transfer between electrical equipment units through the communication nodes. If electric power is always supplied to these nodes, however, power consumption increases, thus increasing loads of a battery and a generator.
In this regard, there have been proposed multiplex communication systems provided with a sleep mode where power supply to communication nodes is suppressed when no data transfer is performed therebetween. In a multiplex communication system of this kind, each communication node determines the necessity of making data transfer between itself and other nodes and enters a sleep mode (more generally, a sleep state) to suppress power consumption when no data transfer is required.
For example, JP-A-9-135257 discloses a multiplex communication system in which each node transmits, when incapable of making a transition to a sleep state, a notification signal to other nodes with which own node can communicate, and on the other hand, each node permits the own node to enter the sleep state, when capable of making a transition to the sleep state and when not receiving a notification signal from any one of the other nodes for a predetermined time period. However, the multiplex communication system having such a construction requires, as a whole, to implement complicated determination processing, because each node individually determines the necessity of transition to the sleep mode.
Another kind of a multiplex communication system having a master node for controlling slave nodes is disclosed in JP-A-2000-32018, for example. This system, which causes slave nodes to wake up from a sleep mode to render them operative, fails to ensure each node of entering a sleep state and does not make a transition of slave nodes to their sleep mode with reliability under collective management.